Hydroformylation or the oxo process is an important large-scale industrial process for preparing aldehydes from olefins, carbon monoxide and hydrogen. These aldehydes can optionally be hydrogenated with hydrogen in the same operation or subsequently in a separate hydrogenation step, to produce the corresponding alcohols. Hydroformylation is carried out in the presence of catalysts which are homogeneously dissolved in the reaction medium. Catalysts used are generally the carbonyl complexes of metals of transition group VIII, in particular Co, Rh, Ir, Pd, Pt or Ru, which may be unmodified or modified with, for example, amine-containing or phosphine-containing ligands. A summarizing account of the processes practiced on a large scale in industry is found in J. Falbe, “New Syntheses with Carbon Monoxide”, Springer Verlag 1980, p. 162 ff.
While short-chain olefins with up to 5 carbon atoms are currently predominantly hydroformylated using ligand-modified rhodium carbonyls as the catalyst, cobalt remains the dominant catalytically active central atom for longer-chained olefins. This is due, firstly, to the high catalytic activity of the cobalt carbonyl catalyst irrespective of the position of the olefinic double bonds, the branch structure and the purity of the olefin to be reacted. Secondly, the cobalt catalyst can be separated off from the hydroformylation products and recycled into the hydroformylation reaction relatively easily. Additionally, catalyst losses during working up can be tolerated more easily owing to the lower price of cobalt.
In one customary process for separating off and recycling the cobalt catalyst, the organic phase of the reactor affluent is freed of cobalt carbonyl complexes by treatment with oxygen or air in the presence of weakly acidic water (cf. DE-AS 24 04 855). In the treatment, the cobalt catalyst is destroyed by oxidation and the central atom is formally converted from the oxidation state −1 to +2 and can then be removed by extraction with the aqueous solution (decobalting). The catalyst complex required for hydroformylation can be re-formed from the cobalt(II) salt solution by reaction with carbon monoxide and hydrogen (carbonyl formation). The re-formed cobalt catalyst is then extracted from the aqueous phase with an organic phase, preferably the olefin to be hydroformylated (catalyst extraction). Besides the olefin, the reaction products and by-products of the hydroformylation can also be used for catalyst extraction. The olefins loaded with the cobalt catalyst are then hydroformylated in a reactor at elevated pressure and elevated temperature (olefin hydroformylation).
The reaction output is customarily withdrawn at the top of the reactor, in particular when a vertical tubular reactor is used. Particularly with higher olefins having 8 carbon atoms or more, the aqueous phase supplied to the reaction zone and necessary to achieve a sufficient catalyst concentration in the reaction zone is not entirely discharged in dissolved or suspended form with the reaction mixture removed from the top. Therefore, reaction output is preferably removed not only at the top, but also from the bottom space.
The uncontrolled or insufficiently controlled withdrawal of aqueous phase from the bottom space leads to operational disruption. An excess of aqueous phase can lead to a decrease in conversion. A deficiency of aqueous phase can lead to local temperature spikes which cause decomposition of the cobalt catalyst.
In the prior art, the amount of reaction output withdrawn from the bottom space is determined in accordance with the level of the aqueous bottom phase in the reactor.
EP 1 204 624 B1 discloses a continuous process for hydroformylation of olefins having 6 to 20 carbon atoms wherein a cobalt catalyst-containing aqueous phase is brought into intimate contact with olefins, hydrogen and carbon monoxide in at least one reaction zone (e.g. in a vertical tubular reactor). Reaction output can be withdrawn not only at the top of the reactor but also from the bottom space of the reactor. The withdrawal of reaction output from the bottom space of the reactor is preferably phase regulated.
EP 1 279 658 B1 discloses a process for hydroformylation of olefins having 5 to 24 carbon atoms to produce the corresponding aldehydes and/or alcohols having 6 to 25 carbon atoms in the presence of unmodified cobalt catalysts in a single-step process wherein the aqueous bottom phase is thoroughly mixed in the reactor with the organic phase, the concentration of cobalt compounds in the aqueous bottom phase is in the range from 0.4 to 1.7% by weight and the level of the aqueous bottom phase is kept constant at a steady state.
However, the level of the aqueous bottom phase often cannot be determined accurately under the conditions prevailing in the reactor.
Radiometric measurement of the level of the aqueous bottom phase is imprecise. The γ-radiation is attenuated by passage through the 30 cm-thick steel walls. Additionally, the calibration of the radiometric measurement is difficult since the level of the aqueous bottom phase in the reactor is, as a rule, not known precisely even at calibration.
Further, the maintenance of apparatuses for determining the level of the aqueous bottom phase is very costly and inconvenient when they are disposed inside the reactor and therefore only accessible at great cost and inconvenience by interrupting the process.
The imprecise determination of the level of the aqueous bottom phase leads to an insufficiently precisely controllable withdrawal of aqueous phase from the bottom space and thereby to operational disruptions that ultimately also lower the yield of crude hydroformylation product.